Image forming apparatus capable of forming a duplex image on one sheet

ABSTRACT

An image forming apparatus, including an image forming part which forms an image on a printing medium, a main feeding part which accommodates the printing medium and supplies the printing medium to the image forming part, and a reversing part which transfers the printing medium, on which an image is formed on a first surface thereof in the image forming part, to the main feeding part, wherein the main feeding part includes a feeding roller part which re-feeds the printing medium, having been transferred through the reversing part, to the image forming part.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/941,255 filed Nov. 16, 2007, which claims the benefit of KoreanPatent Application No. 10-2006-115506, filed Nov. 21, 2006 in the KoreanIntellectual Property Office, the disclosures of which are incorporatedherein by reference.

BACKGROUND

1. Field

Aspects of the present invention relate to an image forming apparatus,and, more particularly, to an image forming apparatus having an improvedfeeding configuration that minimizes a size of a main body of the imageforming apparatus.

2. Description of the Related Art

An image forming apparatus prints image data on a printing medium, suchas paper, according to a printing signal applied from a host apparatussuch as a personal computer (PC). In general, the image formingapparatus comprises a supplying part which supplies individual sheets ofprinting media, an image forming part which forms an image on a printingmedium fed from the supplying part by a feeding part, and a dischargingpart which discharges the image-formed printing medium. Recently, as newprinting technology has been developed, image forming apparatuses alsohave been made to comprise a duplex printing function in which imagesare printed on both sides of the printing medium.

FIG. 1 is a sectional view schematically illustrating a configuration ofan image forming apparatus 10 having a conventional duplex printingfunction. As shown in FIG. 1, the conventional image forming apparatus10 comprises a feeding cassette 20 in which printing media are stored,an image forming part 30 which forms an image on a printing medium fedfrom the feeding cassette 20, a discharging part 40 through which theprinting medium is discharged after the image is formed thereon, and areversing part 50 which changes a feeding direction of the printingmedium discharged through the discharging part 40 and re-feeds the sameprinting medium to the image forming part 30.

However, in the conventional image forming apparatus 10, the reversingpart 50, which reverses the feeding direction of the printing mediumfrom the discharging part 40 to transfer the printing medium toward thefeeding cassette 20, is separately provided on an upper side of thefeeding cassette 20. Accordingly, since the reversing part 50 requires aspace with a predetermined height h1 inside a main frame 60, the entireheight of the image forming apparatus 10 is determined by the sum of theheight h1 of the reversing part 50 and the height h2 of the feedingcassette 20. Accordingly, the height of the reversing part 50 needs tobe minimized so as to reduce the height and the size of the imageforming apparatus 10.

SUMMARY

Accordingly, it is an aspect of the present invention to provide animage forming apparatus with an improved configuration of a reversingpart, and which minimizes the size of a main body thereof.

The foregoing and/or other aspects of the present invention can beachieved by providing an image forming apparatus, including an imageforming part which forms an image on a printing medium, a main feedingpart which accommodates the printing medium and supplies the printingmedium to the image forming part, and a reversing part which transfersthe printing medium, on which an image is formed on a first surfacethereof in the image forming part, to the main feeding part, wherein themain feeding part includes a feeding roller part which re-feeds theprinting medium, having been transferred through the reversing part, tothe image forming part.

According to an aspect of the invention, the image forming apparatusfurther comprises: a casing which accommodates the image forming part;and a sub-feeding part outside of the casing and which supplies theprinting medium to an interior of the casing, wherein the feeding rollerpart transfers the printing medium supplied by the sub-feeding part tothe image forming part.

According to an aspect of the invention, the main feeding part comprisesa main feeding part main body which accommodates the feeding roller partand which is detachably mounted to the casing; and a main feedingcassette part which is spaced from the main feeding part main body andwhich stores the printing medium.

According to an aspect of the invention, the main feeding cassette partcomprises: a knock-up plate in which the printing medium is stored; amain pick-up roller which picks up the printing medium in the knock-upplate; and a registration roller which aligns a leading edge of theprinting medium picked up in the main pick-up roller and which transfersthe printing medium to the image forming part.

According to an aspect of the invention, the feeding roller partcomprises: a sub pick-up roller which picks up the printing medium inthe sub-feeding part; and a feeding roller which transfers the printingmedium transferred to the reversing part and the printing medium pickedup by the sub pick-up roller to the image forming part.

According to an aspect of the invention, the feeding roller partcomprises a second guide member which is provided on one side of thefeeding roller and guides the printing medium guided through the firstguide member to the feeding roller.

According to an aspect of the invention, the feeding roller feeds theprinting medium transferred to the reversing path and the printingmedium picked up in the sub pick-up roller to the registration roller.

Additional aspects and/or advantages of the invention will be set forthin part in the description which follows and, in part, will be obviousfrom the description, or may be learned by practice of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

These and/or other aspects and advantages of the invention will becomeapparent and more readily appreciated from the following description ofthe embodiments, taken in conjunction with the accompanying drawings ofwhich:

FIG. 1 is a sectional view schematically illustrating a configuration ofa conventional image forming apparatus;

FIG. 2 is a perspective view illustrating an exterior configuration ofan image forming apparatus according to an example embodiment of thepresent invention;

FIG. 3 is a sectional view illustrating an interior configuration of animage forming apparatus according to an example embodiment of thepresent invention; and

FIG. 4 is a schematic view illustrating a feeding configuration of animage forming apparatus according to an example embodiment of thepresent invention.

DETAILED DESCRIPTION

Reference will now be made in detail to the present embodiments of thepresent invention, examples of which are illustrated in the accompanyingdrawings, wherein like reference numerals refer to the like elementsthroughout. The embodiments are described below in order to explain thepresent invention by referring to the figures.

FIG. 2 is a perspective view illustrating an exterior configuration ofan image forming apparatus 1 according to an example embodiment of thepresent invention, and FIG. 3 is a sectional view illustrating aninterior configuration of an image forming apparatus 1 according to anexample embodiment of the present invention. Such an image formingapparatus can be a printer, a photo-copier, a facsimile machine or amulti-functional product. As shown in FIGS. 2 and 3, the image formingapparatus 1 comprises a main feeding part 100, a sub-feeding part 200,an image forming part 300, a fusing part 400, a discharging part 500, areversing part 600, and a main body 700 which supports all thecomponents.

The main feeding part 100 transfers a printing medium, such as paper, P0stored in a main feeding cassette part 120 or in a sub-feeding part 200.The main feeding part 100 also transfers a printing medium P1, having animage formed on a first surface thereof, from a reversing part 600 to animage forming part 300. The image forming part 300 forms images on theprinting media P0 and P1. The fusing part 400 fuses the images formed inthe image forming part 300. The discharging part 500 discharges theprinting medium from the fusing part 400. The reversing part 600re-feeds the printing medium P1 toward a feeding roller part 130according to a selection of a user to initiate a duplex printingoperation. The main body 700 has a main frame 710 supporting theabove-described components.

Hereinafter, the reference numeral P0 refers to a printing medium onwhich no image is formed. Conversely, the reference numeral P1 refers tothe same printing medium on which an image is formed on one sidethereof. The reference numeral P2 refers to the same printing medium onwhich an image is formed on both sides thereof.

The main feeding part 100 according to an example embodiment of thepresent invention comprises a main feeding part main body 110 that isattachable and/or detachable with respect to the main body 700. A mainfeeding cassette part 120 is provided on an opposite side of a feedinginlet 111 of the main feeding part main body 110 and stores the printingmedium P0. The feeding roller part 130 transfers the printing medium P1from the reversing part 600 and the printing medium P0 transferred fromthe sub-feeding part 200 to the image forming part 300.

As shown in FIGS. 2 and 3, the main feeding part main body 110 isdetachably mounted to the main body 700. The main feeding part main body110 is mounted to the main body 700 and transfers the printing medium P0to the image forming part 300 according to a receipt of a printingsignal. Also, if the supply of the printing medium P0 is exhausted or ajam occurs, the main feeding part main body 110 may be separated fromthe main body 700 to allow for a loading of a printing medium P0 or toallow for a solution to the jam.

The main feeding part main body 110 has a feeding inlet 111 that allowsfor the feeding of the printing medium P0 in the sub-feeding part 200inside the main body 700. If the sub-feeding part 200 is pivotallyrotated from the main body 700 to a feeding position, the feeding inlet111 allows the printing medium P0 of the sub-feeding part 200 to contacta sub pick-up roller 131.

The main feeding cassette part 120 is provided in a rear area of themain feeding part main body 110 and supplies the stored printing mediumP0 to the image forming part 300. The main feeding cassette part 120comprises a knock-up plate 121 on which printing media, including theprinting medium P0, are stacked. A main pick-up roller 123 separates theprinting medium P0 from the stack. A registration roller 125 aligns aleading edge of both the picked up printing medium P0 and the printingmedium P1. The registration roller 125 also supplies the printing mediumP0 and the printing medium P1 to the image forming part 300. Theknock-up plate 121 is biased to move toward the main pick-up roller 123and enables the printing medium P0 stored on the top of the stack ofprinting media to contact the main pick-up roller 123 if the mainfeeding part 100 is mounted to the main body 700.

The main pick-up roller 123 rotates in contact with the printing mediumP0 stored on the top of the stack of printing media on the knock-upplate 121 if the printing signal is applied. The main pick-up roller 123then separates the printing medium P0 from the stack by a frictionalforce generated between the printing medium P0 and the main pick-uproller 123 when rotating. At this time, the main pick-up roller 123applies a larger frictional force than that which is generated betweenthe printing medium P0 on the top of the stack and the next printingmedium in the stack. This prevents an overlapped transfer of theprinting medium P0.

The registration roller 125 aligns the leading edge of the printingmedium P0, having been separated from the knock-up plate 121, andsupplies the printing medium P0 to a transfer belt 341 of the imageforming part 300. The registration roller 125 may be a center alignmenttype of registration roller, which aligns the printing medium P0 from acentral area of the leading edge of the printing medium P0 to a sidearea, and a side alignment type of registration roller, which aligns theprinting medium P0 from a side area of the leading edge of the printingmedium P0. Further description of the configuration of the registrationroller 125 will be omitted as it is a known technology.

The feeding roller part 130 comprises a sub pick-up roller 131, whichseparates the printing medium P0 stored in the sub-feeding part 200 fromother printing media. A feeding roller 133 transfers the printing mediumP0 separated by the sub pick-up roller 131 and the printing medium P1 onwhich an image is formed on a first surface thereof and which istransferred from the printing medium reversing part 600 to theregistration roller 125. A second guide member 135 guides the printingmedium P1 transferred from the printing medium reversing part 600 to thefeeding roller 133.

The sub pick-up roller 131 is provided on one side of the feeding inlet111 and picks up a printing medium at a top portion of the sub-feedingpart 200 that is proximate to the inside of the main body 700. Thedescription of the sub pick-up roller 131 will be omitted as thisfeature has the same configuration as that of the main pick-up roller123.

The feeding roller 133 feeds the printing medium P0 separated by the subpick-up roller 131 to the registration roller 125, and changes thefeeding direction of the printing medium P1 on which an image is formedon a first surface thereof and which is fed through the reversing part600 to transfer the printing medium P1 to the registration roller 125. Afeeding direction of the printing medium P1 is changed toward the imageforming part 300 by the feeding roller 133 to allow for the secondsurface of the printing medium P1 to contact each of the developingdevices 310Y, 310M, 310C, and 310K.

The feeding roller 133 comprises a driving roller 133 a which is rotatedby a driving part (not shown), and a driven roller 133 b which isrotated with the rotation of the driving roller 133 a. The feedingroller 133 may be provided as a singular roller, or may be provided as aplurality of rollers according to a distance and a path from the subpick-up roller 131 to the registration roller 125.

The second guide member 135 is provided on one side of the feedingroller 133 and guides the printing medium P1 to the feeding roller 133.The feeding roller 133 is provided inside the main feeding part 100 tobe detachably mounted to the main body 700 and to be separate from areversing path 711 (see FIGS. 3 and 4) provided in the main body frame710. However, the printing medium P1 tends to move in a substantiallystraight proceeding direction as a result of the rigidity thereof.Accordingly, the second guide member 135 guides the printing medium P1,having passed through the reversing path 711 to the feeding roller 133,to enable the feeding direction of the printing medium P1 to be changedin a stable manner.

The second guide member 135 may have a length and a shape that eachcorrespond to a portion of the space from one end part of the reversingpath 711 to the feeding roller 133. Also, the second guide member 135may be straight or curved. The second guide member 135 may comprise apair of members to guide both a front surface and a rear surface of theprinting medium. The second guide member 135 may be adhered on one sideof the feeding roller 133 as a separate member, or may be integratedwith the main feeding part main body 110.

The sub-feeding part 200 is coupled to a casing 750 of the main body 700to allow the sub feeding part 200 to be pivotally rotated and may storeprinting media having different sizes or characteristics (i.e., OHPfilm, photographic paper, scratch paper, or B4 and B5 size paper) thanthe printing medium P0. The sub-feeding part 200 is coupled to thecasing 750 of the main body 700 as shown in FIG. 2 if a user does notwant to use it. Also, the sub-feeding part 200 is pivotally rotated in apredetermined angle with respect to the casing 750 as shown in FIG. 3 tosupply the printing medium P0 to the feeding inlet 111 if the user doeswant to use it.

The image forming part 300 forms an image on a first surface of theprinting medium P0 fed from the main feeding part 100 and thesub-feeding part 200, and forms an image on the second surface of theprinting medium P1. The image forming part 300 may be an ink jet whichforms an image by ejecting ink onto the printing medium, anelectrophotographic type which selectively spreads developer on theprinting medium P0 or P1 according to a potential difference between aphotosensitive body and a developer, and a direct thermal type whichapplies heat and pressure to an ink ribbon on which ink is coated andtransfers the ink to the printing medium P0 or P1 to form an imagethereon.

The image forming part 300 according to an example embodiment of thepresent invention employs a color electrophotographic method. The imageforming part 300 comprises four developing devices 310Y, 310M, 310C, and310K that are provided to form yellow, magenta, cyan, and black imagesto form a full color image in a single pass of the printing medium P0 orP1 through the image forming part 300.

The plurality of developing devices 310Y, 310M, 310C, and 310K spreaddeveloper T of each color onto the printing media P0 and P1. An exposurepart 320 scans light on the surface of photosensitive bodies 311Y, 311M,311C, and 311K of each of the developing devices 310Y, 310M, 310C, and310K, respectively, to form an electrostatic latent image thereon. Aplurality of transfer rollers 330Y, 330M, 330C, and 330K transfer thedeveloper spread onto the photosensitive bodies 311Y, 311M, 311C, and311K to the printing media P0 and P1. A feeding part 340 sequentiallytransfers the printing media P0 and P1 to the plurality of developingdevices 310Y, 310M, 310C, and 310K.

The plurality of developing devices 310Y, 310M, 310C, and 310K comprisethe photosensitive bodies 311Y, 311M, 311C, and 311K, developer storingparts 317Y, 317M, 317C, and 317K which accommodate developer T therein,developing rollers 313Y, 313M, 313C, and 313K which develop thedeveloper on an electrostatic latent image of the photosensitive bodies311Y, 311M, 311C, and 311K, supplying rollers 314Y, 314M, 314C, and 314Kwhich supply the developer T for the developing rollers 313Y, 313M,313C, and 313K, and electrifying rollers 315Y, 315M, 315C, and 315Kwhich electrify the photosensitive bodies 311Y, 311M, 311C, and 311K toa predetermined potential. The description of a configuration of thedeveloping devices 310Y, 310M, 310C, and 310K will be omitted as it issimilar to that of the conventional configuration.

The exposure part 320 scans a beam onto each of the photosensitivebodies 311Y, 311M, 311C, and 311K provided in each of the developingdevices 310Y, 310M, 310C, and 310K to form an electrostatic latent imageon each of the bodies. The exposure part 320 has a multi-beam lightscanning configuration to allow the exposure part 320 to scan the beamto the plurality of photosensitive bodies 311Y, 311M, 311C, and 311Ksimultaneously. The exposure part 320 comprises a light source (notshown), a polygon mirror 321 which cause the beam to be inclined, and anf-Θ. lens 325 which scans the beam inclined in the polygon mirror 321onto a surface to be scanned to form an image. The light source (notshown) may have a plurality of luminosities, or may be provided with asemiconductor element having a single luminosity to correspond to therespective colors. As shown in FIG. 3, the polygon mirror 321 isprovided as a pair of mirrors, and the pair allows two beams to bescanned onto different paths. The f-Θ lens 325 along with a lens isprovided on each of the four paths. Accordingly, the f-Θ lens 325 canseparately scan the beam with respect to the plurality of photosensitivebodies 311Y, 311M, 311C, and 311K which are disposed to be adjacent eachother.

The transfer rollers 330Y, 330M, 330C, and 330K face each of thephotosensitive bodies 311Y, 311M, 311C, and 311K across the printingmedia P0 and P1 which are transferred along a transfer belt (PTB). Also,the transfer rollers 330Y, 330M, 330C, and 330K apply a predeterminedtransfer voltage to rear surfaces of the printing media P0 and P1 totransfer the developer spread on the surface of the photosensitivebodies 311Y, 311M, 311C, and 311K to the printing media P0 and P1. Thetransfer rollers 330Y, 330M, 330C, and 330K recognize a resistance valueof the printing media as having different values according to thethickness and the material of the transferred printing media and applyan optimum transfer voltage corresponding to the resistance value.

The feeding part 340 feeds the printing media P0 and P1 to sequentiallyspread the developer from each of the photosensitive bodies 311Y, 311M,311C, and 311K to form an image on the printing media P0 and P1. Thefeeding part 340 comprises a transfer belt (PTP) which adsorbs theprinting medium whose leading edge is aligned in the registration roller125 onto a surface thereof by static electricity and rotates, a beltdriving roller 343 which drives the rotation of the transfer belt 341,and a belt electrifying roller 345 which electrifies the surface of thetransfer belt 341. The description of the configuration of the feedingpart 340 will be omitted as it is similar to that of a conventionalconfiguration thereof.

The fusing part 400 fuses the developer on the surface of the printingmedium P1 whose first surface is previously printed on and printingmedium P2 whose second surface is previously printed on by applying heatand pressure to the printing media P0, P1, and P2. The fusing part 400comprises a heating roller 410 and a pressing roller 420 which applyheat and pressure, respectively, to the printing media P0, P1 and P2.

The discharging part 500 discharges the image-formed printing media P0,P1 and P2 through the fusing process in the fusing part 400 to a storingpart 753 of the casing 750. The discharging part 500 comprises adischarging roller 510 which discharges the printing media P0, P1 andP2, and a reverse roller 520 which changes a feeding direction of theprinting medium P1 according to a user's duplex printing signal andtransfers the printing medium P1 to the printing medium reversing part600. Also, the discharging part 500 may further comprise a reversinglever (not shown) which guides the printing medium P1 to the printingmedium reversing part 600.

The reversing part 600 transfers the printing medium P1 from the reverseroller 520 and toward the feeding roller 133 of the main feeding part100. The reversing part 600 comprises a reversing roller 610 providedalong the reversing path 711 formed in the main body frame 710. Thereversing roller 610 may comprise a plurality of rollers according tothe length and the path of the reversing path 711.

The reversing part 600 also comprises a first guide member 620 whichstably guides the printing medium P1 transferred through the paperreversing path 711 to the feeding roller 133 in an outlet area of thereversing path 711. The first guide member 620 may comprise a pair ofmembers on opposite sides of the reversing path 711.

The main body 700 comprises a main body frame 710 supporting theabove-described components, and a main body casing 750 to protect themain body frame 710 and the components from an external impact. The mainbody frame 710 includes the reversing path 711 to guide the printingmedium P1 to be transferred to the feeding roller 133 of the mainfeeding part 100. The reversing path 711 may have a length correspondingto the height of the plurality of developing devices 310Y, 310M, 310C,and 310K.

The main body casing 750 is coupled to the sub-feeding part 200. Themain body casing 750 comprises a storing part 753 which stores theprinting media P1 and P2.

Hereinafter, a process for use with the image forming apparatus 1 willbe described.

First, a one-sided printing process in which a user forms an image onone surface of the printing medium P0 stored in the main feedingcassette part 120 (see A in FIG. 4) will be described. If a printingsignal is applied, the main pick-up roller 123 is rotated to pick up theprinting medium P0. The picked-up printing medium P0 is aligned in theregistration roller 125 to be transferred to the image forming part 300.

The transfer belt 341 applies a static electric force to the printingmedium P0 electrified by the belt electrifying roller 345 andtransferred from the registration roller 125 to adsorb the printingmedium P0 on its surface. As the belt driving rollers 343 a and 343 bare driven, the transfer belt 341 is rotated and the printing medium P0sequentially contacts each of the photosensitive bodies 311Y, 311M,311C, and 311K. At this time, the developer of each color is spread onthe surface of the printing medium P0 to form an image.

The fusing part 400 fuses the developer by an application of heat andpressure. Accordingly, the printing medium P0, on which an image isformed on its first surface and which is now referred to as printingmedium P1, is discharged by the discharging roller 510.

If the user selects duplex printing (see B in FIG. 4), the procession ofthe printing medium P1 is reversed by the reverse roller 520 of thedischarging roller 510 to re-enter the main body 700. Also, the printingmedium P1 is guided by a direction changing lever (not shown) to enterthe reversing path 711. The printing medium P1 having entered thereversing path 711 is then transferred by the reversing roller 610 andguided to the second guide member 135 by the first guide member 620 tothen be guided to the feeding roller 133.

The direction of the feeding of the printing medium P1 is changed by thefeeding roller 133, and, at this time, the second surface of theprinting medium P1 is caused to face the photosensitive bodies 311Y,311M, 311C, and 311K. The printing medium P1 passes through theregistration roller 125 to be aligned, and an image is then formed onthe second surface in the image forming part 300. Then, the printingmedium P2 is discharged through the discharging part 500.

Meanwhile, an image forming process of the printing medium P0 stored inthe sub-feeding part 200 will be described by referring to C in FIG. 4.The printing medium P0 stored in the sub-feeding part 200 enters, by therotation of the sub pick-up roller 131, the inside of the main body 700through the feeding inlet 111. The printing medium P0 is then guidedthrough the feeding roller 133 to the registration roller 125, andpasses through the image forming part 300 to have an image-formedthereon.

As is described above, in the image forming apparatus according toaspects of the present invention, a feeding roller which has beenprovided separately in a conventional duplex printing part is, accordingto an aspect of the present invention, provided in the inside of a mainfeeding part to thereby minimize the height of the duplex printing part.Also, the number of the feeding rollers provided in the duplex printingpart and the main feeding part may be reduced to lower production costs.

Also, in the image forming apparatus, according to aspects of thepresent invention, a feeding roller part to feed a printing medium ofthe sub-feeding part is integrated with the main feeding part to savespace and to minimize the size of the image forming apparatus. The imageforming apparatus, according to aspects of the present invention hasemployed a single-pass color electrophotographic type, but it may beapplied to a multi-pass color electrophotographic type, and a monoelectrophotographic type. Also, the image forming apparatus according toaspects of the present invention may be applied to a multi-functionprinter comprising a scanner and a facsimile.

As is described above, the image forming apparatus, according to aspectsof the present invention, provides at least the following two effects.First, a feeding roller part to feed a printing medium, of a duplexprinting part and a main feeding part, is integrated with the mainfeeding part to minimize the size of the main body. Second, since theduplex printing part and the sub-feeding part share one feeding roller,it has a simple configuration and less components, thereby savingproduction costs and improving an assembling configuration.

Although a few embodiments of the present invention have been shown anddescribed, it would be appreciated by those skilled in the art thatchanges may be made in these embodiments without departing from theprinciples and spirit of the invention, the scope of which is defined inthe claims and their equivalents.

What is claimed is:
 1. An image forming apparatus, comprising: an imageforming part which forms an image on a printing medium; a main feedingpart which accommodates the printing medium and supplies the printingmedium to the image forming part; and a reversing part which transfersthe printing medium, on which an image is formed on a first surfacethereof in the image forming part, to the main feeding part, wherein themain feeding part includes a feeding roller part which re-feeds theprinting medium, having been transferred through the reversing part, tothe image forming part.
 2. The image forming apparatus according toclaim 1, further comprising: a casing which accommodates the imageforming part; and a sub-feeding part outside of the casing and whichsupplies the printing medium to an interior of the casing, wherein thefeeding roller part transfers the printing medium supplied by thesub-feeding part to the image forming part.
 3. The image formingapparatus according to claim 2, wherein the main feeding part comprises:a main feeding part main body which accommodates the feeding roller partand which is detachably mounted to the casing; and a main feedingcassette part which is spaced from the main feeding part main body andwhich stores the printing medium.
 4. The image forming apparatusaccording to claim 3, wherein the main feeding cassette part comprises:a knock-up plate in which the printing medium is stored; a main pick-uproller which picks up the printing medium in the knock-up plate; and aregistration roller which aligns a leading edge of the printing mediumpicked up in the main pick-up roller and which transfers the printingmedium to the image forming part.
 5. The image forming apparatusaccording to claim 4, wherein the feeding roller part comprises: a subpick-up roller which picks up the printing medium in the sub-feedingpart; and a feeding roller which transfers the printing mediumtransferred to the reversing part and the printing medium picked up bythe sub pick-up roller to the image forming part.
 6. The image formingapparatus according to claim 5, wherein the reversing part comprises: areversing path which is provided in the casing and on which the printingmedium is transferred; and a reversing roller which is provided on thereversing path and which transfers the printing medium, wherein, a firstguide member which guides the printing medium to the feeding roller isprovided in one end part of the reversing path.
 7. The image formingapparatus according to claim 6, wherein the feeding roller partcomprises a second guide member which is provided on one side of thefeeding roller and which guides the printing medium, the printing mediumhaving been guided through the first guide member to the feeding roller.8. The image forming apparatus according to claim 5, wherein the feedingroller feeds the printing medium, the printing medium having beentransferred to the reversing path and the printing medium having beenpicked up in the sub pick-up roller to the registration roller.
 9. Animage forming apparatus, comprising: an image forming unit to form animage on a printing medium; a main feeding unit to supply the printingmedium to the image forming unit; a reversing unit to transfer theprinting medium, on which an image is formed on a first surface thereofin the image forming unit, to the main feeding unit; and a feedingroller installed in the main feeding unit to re-feed the printingmedium, having been transferred through the reversing unit, to the imageforming unit.
 10. The image forming apparatus according to claim 1,further comprising: a casing to accommodate the image forming unit; anda sub-feeding unit, outside of the casing, to supply the printing mediumto an interior of the casing, wherein the feeding roller transfers theprinting medium supplied by the sub-feeding unit to the image formingunit.
 11. The image forming apparatus according to claim 10, wherein themain feeding unit comprises: a main body to accommodate the feedingroller and to detachably mount to the casing; and a main feedingcassette, spaced from the main body, to store the printing medium. 12.The image forming apparatus according to claim 11, wherein the mainfeeding cassette comprises: a knock-up plate to store the printingmedium; a pick-up roller to pick up the printing medium from theknock-up plate; and a registration roller to align a leading edge of thepicked up printing medium and to transfer the printing medium to theimage forming unit.
 13. The image forming apparatus according to claim12, wherein the feeding roller comprises: a sub pick-up roller to pickup the printing medium from the sub-feeding unit; and a feeding rollerto transfer the printing medium, having been transferred to thereversing unit, and the printing medium, having been picked up by thesub pick-up roller, to the image forming unit.
 14. The image formingapparatus according to claim 13, wherein the reversing unit comprises: areversing path, provided in the casing, on which the printing medium istransferred; a reversing roller, provided on the reversing path, totransfer the printing medium; and a first guide member to guide theprinting medium to the feeding roller, the first guide member beingprovided in one end part of the reversing path.
 15. The image formingapparatus according to claim 14, wherein the feeding roller comprises asecond guide member, provided on one side of the feeding roller, toguide the printing medium, the printing medium having been guidedthrough the first guide member to the feeding roller.
 16. The imageforming apparatus according to claim 13, wherein the feeding rollerfeeds the printing medium, the printing medium having been transferredto the reversing path and the printing medium having been picked up inthe sub pick-up roller to the registration roller.